Siliciclastic Progradation Within a Neogene Carbonate Passive Margin - Northern Carnarvon Basin of the Northwest Shelf of Australia

Interpretations of extensive industry subsurface data (3D and 2D seismic data, wireline logs and completion reports) and results from previous studies suggest that relative sea level changes and ocean currents controlled observed variations in carbonate vs. siliciclastic sediment type and stratal architecture in the Northern Carnarvon Basin, Northwest Shelf of Australia, during the Neogene. The basin has been a site of predominantly carbonate sedimentation since the late Paleogene. However, significant (at least 30-70 km) siliciclastic progradation on top of this carbonate shelf started in the late middle Miocene. Carbonate- dominated sedimentation resumed in the late Miocene/Pliocene. Siliciclastic-rich seismic packages show dip- oriented progradation northwestward, as well as strike-oriented progradation northeastward. Clinoform heights, up to approx. 140 m, and a dip-elongated lobe morphology suggest deposition by a long-lived (approx. 6 My) deltaic system which is no longer active on this margin. Consistent northeastward switching of these interpreted delta lobes and their progradation over the preexisting carbonate shelf sediments suggest wave-dominated conditions, with strong northeastward long-shore currents, for this delta system during a time of relative base level fall. During this siliciclastic progradation event, carbonate sedimentation continued in the updrift direction southwest of the delta, but carbonates were not accumulated downdrift or northeast, where progressively younger deltaic siliciclastics filled available accommodation space. By the beginning of the Pliocene, siliciclastic sedimentation had retreated towards the Australian coast, allowing widespread carbonate production to resume; interpreted shallow-water carbonate platforms then developed over subtle topographic highs created by the underlying deltaic lobes.